1. Field of the Invention
The present invention relates to a wireless local area network (WLAN), and more particularly, to methods and apparatuses for a station, permitted to operate within available channels not used by a licensed device, to select a channel to begin its transmission in a Wireless Local Area Network (WLAN).
2. Discussion of the Related Art
The standard for a Wireless Local Area Network (WLAN) technology is established by IEEE 802.11 standard association. IEEE 802.11a/b among IEEE 802.11 standards provides 11 Mbps (IEEE 802.11b) or 54 Mbps (IEEE 802.11a) transmission efficiency using unlicensed band on 2.4. GHz or 5 GHz frequency band. IEEE 802.11g, adapting OFDM (Orthogonal Frequency Divisional Multiplexing) technology, provides 54 Mbps transmission efficiency. And, IEEE 802.11n, adapting MIMO-OFDM technology, provides 300 Mbps transmission efficiency for 4 spatial streams. IEEE 802.11n provides 40 MHz channel bandwidth, and in this case it provides up to 600 Mbps transmission efficiency.
Now, a standard for regulating the WLAN operation in TV White Space is under establishment, as IEEE 802.11 af.
TV Whitespace includes channels allocated to broadcast TV, which are permitted to be used by cognitive radio device. TV White Space may include UHF band and VHF band. The spectrum not used by a licensed device (hereinafter, can be called as ‘White Space’) can be used by an unlicensed device. The frequency band permitted to be used by unlicensed device can be differently defined for each country. Generally, this frequency band comprises 54-698 MHz (US, Korea), and some of this frequency band can't be used for the unlicensed device. Here, ‘licensed device’ means a device of the user permitted in this frequency band, and can be differently called as ‘primary user’, or ‘incumbent user’. Hereinafter, the term of ‘incumbent user’ can be collectively used for these terms.
Unlicensed device should provide a protection mechanism for the incumbent user. That is, the unlicensed device should stop using a specific channel, when an incumbent user, such as wireless microphone, is using that specific channel. For this purpose, spectrum sensing mechanism is required. Spectrum sensing mechanism comprises Energy Detection scheme, Feature Detection scheme, etc. By using this mechanism, unlicensed device determines that the channel is used by an incumbent user, when the strength of the primary signal is greater than a predetermined level, or when DTV (Digital Television) Preamble is detected. And, the unlicensed device (station or Access Point) shall lower its transmission power, when it is detected that the neighboring channel, next to the channel used by the unlicensed device, is used by the incumbent user.
Further, the unlicensed device should search for a network to be connected in a specific channel in WS (White Space). As stated above, the unlicensed device is permitted to operate only within available channels not used by the incumbent user.
For these purposes, the unlicensed device, which wishes to use the White Space (WS), shall acquire information for available channel list at its location. However, individual sensing mechanism takes significant time and consume huge amount of power, thus it is needed for the unlicensed device to acquire the available channel information more effectively.
The unlicensed device should be enabled by an enabling STA or enabler to operate in WS. And, for some times, a STA operating as a non-AP station has to operate as an AP STA. Therefore, appropriate mechanisms for these operations are needed.